The invention relates to a so-called chip-on-chip (hereinafter abbreviated as COC) type semiconductor device, in which a plurality of semiconductor chips is electrically interconnected as opposed to each other, and a method for manufacturing the same. More particularly, the invention relates to a semiconductor device having such a construction that two mutually opposed semiconductor chips thereof can be joined to each other without being damaged due to a high temperature applied thereon or mechanical shock by supersonic waves when they are connected to each other and a method for manufacturing the same.
Conventionally, there has in some cases been used a so-called a COC (chip on chip) type semiconductor device having a construction of composing a semiconductor circuit of a plurality of chips by interconnecting them one on the other for a purpose of, for example, reducing an occupied area by means of three-dimensioning or standardizing part of the circuit (for example, changing driver circuits based on application by standardizing the memory portions), if the semiconductor device is composed of a plurality of circuits like in the case of a combination of memory portions and the corresponding driver circuits.
As shown, for example, in FIG. 17 showing a process of joining two semiconductor chips 1 and 2, a semiconductor device having the above-mentioned construction is fabricated by fixing the one semiconductor chip 1 on a heated board stage 51 and the other semiconductor chip 2 to a mount head 52, pressing the mount head 52 to permit bump electrodes 11 and 21 made of Au etc. of the respective chips to come in contact with each other under pressure, and heating it to a temperature of 450xc2x0 C. or so to interconnect these bump electrodes 11 and 21 electrically. Note here that, as mentioned above, the bumps 11 and 21 are made of a metal material such as Au having a higher melting point than solder because the semiconductor device is mounted on the mother board by soldering.
Since the above-mentioned COC type semiconductor device employs high melting point metal such as Au as the material of its bump electrodes, to interconnect a plurality of semiconductor chips thereof, it is necessary to heat them to a temperature of 450xc2x0 C. or so under pressure in order to obtain good electrical interconnection. When the chips are heated to such a high temperature during interconnection, the semiconductor substrate is also heated to a temperature of 450xc2x0 C. or higher, so that the circuit elements formed in the semiconductor substrate (elements such as transistors constituting the semiconductor device) are heated to a high temperature, thus fluctuating in properties, which is a problem. Further, although these chips can be joined to each other at a temperature of 450xc2x0 C. or so, if under pressure as mentioned above, for example, once joined to each other, they cannot be separated without being destroyed in most cases because Au has a high melting point.
In view of the above, it is an object of the present invention to provide such a COC type semiconductor device and a method for manufacturing the same that has a construction which is capable of interconnecting the electrodes of semiconductor chips without being affected by a temperature at which the semiconductor device is mounted and also without deteriorating the properties of the semiconductor chips owing to a high temperature applied thereon.
It is another object of the present invention to provide a semiconductor device having a construction which is capable of securely connecting a bump electrode of a semiconductor chip, even if small, to a partner semiconductor chip.
It is a further object of the present invention to provide a semiconductor device having a construction which is capable of securely connecting each other in the event that one is a wiring.
It is a still further object of the present invention to provide a semiconductor device having a construction which is capable of securely interconnecting a parent chip and a child chip and separating from each other easily without affecting elements in the semiconductor chips when the child chip is removed.
It is an additional object of the present invention to provide a semiconductor chip having such a construction that avoids force from being concentrated on the bump electrode at a joining portion so that the element formed in a semiconductor layer underlying the joining portion may not be affected adversely.
It is another additional object of the present invention to provide a semiconductor manufacturing method which is capable of joining the bump electrodes at a relatively lower temperature and also easily without accurate alignment thereof.
A semiconductor device according to the present invention includes; a first semiconductor chip having an electrode terminal, a second semiconductor chip having an electrode terminal, a bump electrode made of a first metal for joining the first and second semiconductor chips, the bump electrode being provided on at least one of the electrode terminal of the first semiconductor chip and the electrode terminal of the second semiconductor chip, and an alloy layer formed on a joining portion where the first and second semiconductor chips are joined with each other via the bump electrode, the alloy layer being made of an alloy of the first metal and a second metal, wherein the second metal is made of such a metal that can melt at a temperature lower than a melting point of the first metal and be alloyed with the first metal.
The first or second metal referred to here includes not only pure metal but also an alloy and composite metal made of at least two kinds of metal as well as main metal on the outer layer side in a case of a stack containing at least two kinds of metal. Also, an alloy layer of the first and second metal includes an entirely alloy layer as well as such a layer that part thereof is an alloy and the other part is made of only the first or second metal or any other compound. Further, the constructions of joining via the bump electrode include a construction in which one bump electrode and another are joined to each other or another in which a bump electrode is provided to the electrode terminal of one semiconductor chip so that it may be directly joined to the electrode terminal of the other semiconductor chip therethrough.
By employing such a construction, it is possible to electrically interconnect first and second semiconductor chips by joining their respective electrode terminals at a relatively low temperature by melting the second metal having a relatively low melting point to alloy it with the first metal which the bump electrode is made of. As a result, the bump electrode can be connected at a low heating temperature almost equal to the melting point of the second metal, so that by selecting, for example, Sn as the material of the second metal, it is unnecessary to apply such a heating temperature that may affect the circuit elements, thus avoiding adverse effects on these elements owing to a high temperature. Moreover, the first metal that makes up the major part of the bump electrode has a relatively high melting point and, therefore, causes no trouble at around the soldering temperature during mounting.
According to another aspect of the semiconductor device of the present invention includes; a first semiconductor chip having an electrode terminal, a second semiconductor chip having an electrode terminal, a bump electrode made of a first metal for joining the first and second semiconductor chips, the bump electrode being provided on at least one of the electrode terminal of the first semiconductor chip and the electrode terminal of the second semiconductor chip, and a third metal layer having a lower melting point than that of the first metal provided on a joining portion where the first and second semiconductor chips are joined with each other via the bump electrode. That is, rather than directly forming an alloy with the bump electrode or the electrode terminal, by providing, for example, the third metal layer having a low melting point such as the alloy formed by the first and second metal, the third metal layer melts at a relatively low temperature and diffuses into the bump electrode or the electrode terminal to thereby provide diffusive joining.
Note here that like the above-mentioned first and second metal, the third metal layer also includes not only pure metal but also an alloy etc., so that after being joined, it may be combined or alloyed with the first or second metal to have an uneven composition. In this case also, although the third metal layer has a relatively low melting point and melts readily at an elevated temperature, it is very thin and mostly supported by the first metal and, in a package, is not peeled off and, at the same time, may be easily peeled off by an external force when heated.
According to a further aspect of the semiconductor device of the present invention includes, a first semiconductor chip having an electrode terminal, a second semiconductor chip having an electrode terminal, a bump electrode made of a first metal for joining the first and second semiconductor chips, the bump electrode being provided on at least one of the electrode terminal of the first semiconductor chip and the electrode terminal of the second semiconductor chip, and a detachable material portion provided on a joining portion where the first and second semiconductor chips are joined with each other via the bump electrode, the detachable material being made of such a material that the first and second semiconductor chips can be easily separated from each other at a temperature of 280xc2x0 C. to 500xc2x0 C.
That is, while the bump is made of metal such as Au having a high melting point, the joining portion is joined to by such metal that melts at a lower temperature, for example, 300xc2x0 C. or lower, so that the metal with a lower melting point provides a very thin layer and can hardly be peeled off at around a temperature of 300xc2x0 C. but be easily peeled off by an external force at around that temperature in construction.
Specifically, in such a construction that the bump electrode is formed on the electrode terminal of each of the first and second semiconductor chips, and such a construction that the bump electrode is formed on the electrode terminal of either one of the first and second semiconductor chip so that a metal film made of the first or second metal may be formed on the electrode terminal of the other semiconductor chip, the alloy layer is formed on the joining portion between these bump electrodes or between the bump electrode and the electrode terminal respectively. Also, at least one of the bump electrodes is made of the first metal, so that it can be alloyed with the second metal provided on the right and side face of the bump electrode to thereby joined to via an alloy layer, or joined to via the third metal layer. And they are joined securely by forming a fillet at the joining portion even with a small size of the bump electrode.
A fillet made of the alloy layer of the first and second metal or of the third metal may be formed on the joining portion between the first and second semiconductor chips, thus highly securing the joining. The fillet here refers to such a portion of the joining portion that protrudes out on the side of the side wall, including a smoothly sloped one as well as a small one protruded only in the vicinity of the joining portion.
More specifically, if the first metal consists of Au and the second metal consists of Sn, or if the third metal consists of an Auxe2x80x94Sn alloy, the joining portion comes to have the Auxe2x80x94Sn alloy, thus making it possible to connect a bump electrode only by applying a relatively low temperature of 300xc2x0 C. or so, at which the circuit elements are not affected.
According to a still further aspect of the semiconductor device of the present invention includes; a first semiconductor chip having an electrode terminal or a wiring, a second semiconductor chip having an electrode terminal or a wiring, and a low-melting point metal layer provided on the surface of the wiring of at least one of the first and second semiconductor chips, wherein the first and second semiconductor chips are electrically interconnected and joined to each other via the low-melting point metal layer so that the electrode terminal or wiring is face to face each other.
In this case, the low-melting point metal layer includes a joining portion which at least partially has an alloy formed thereon due to alloying of the first and second metal or a boundary portion which is joined in a diffusive manner owing to the provision of the third metal.
To flatten the wiring surface, the wiring can be formed via a second insulating layer onto a passivation film on the semiconductor chip, thus joining the joining portion in secure contact even if the bump electrode of the joining partner semiconductor chip or even if the wiring portions are directly joined to each other without bump electrodes.
The first and second semiconductor chips are joined to each other between their respective wirings, and the low-melting point metal layer is provided on a joining portion, and further including a first insulating layer provided as interposed at a gap between the a couple of wirings except at the joining portion, thus securely interconnecting the wiring with no possible contact between the wiring provided on the outermost surface.
The wiring can be made of an Au via a barrier metal layer provided to the electrode terminal as connected thereto, and the low-melting point metal layer can be made of an Auxe2x80x94Sn alloy. Further the wiring can include a Cu wiring made of Cu formed simultaneously with the electrode terminal, a barrier metal layer provided on the Cu wiring, and an Au wiring provided on the barrier metal layer, wherein the low-melting point metal layer is made of an Auxe2x80x94Sn alloy and is provided on the Au wire. Further the wiring can be made of Au formed simultaneously with the electrode terminal, and the low-melting point metal layer can be made of an Auxe2x80x94Sn alloy.
By forming the Auxe2x80x94Sn alloy constituting the joining portion so that it may be rich in Au by containing Au by 65 weight-percent or more, an eutectic alloy can be obtained, so that the joining portion can be connected securely and still can be easily removed by heating it to a temperature of 300xc2x0 C. or so because the eutectic alloy has a low melting point. Further, the joining strength can be stabilized even more if the Auxe2x80x94Sn alloy layer of the joining portion is 0.8 xcexcm or more and 5 xcexcm or less.
Also, if the gap in the joining portion between the first and second semiconductor chips is filled with an insulating resin having nearly the same elastic modulus as the bump electrode, even though a compressing force due to, for example, shrinkage of the resin package is applied on these chips, the pressure on the semiconductor layer is not concentrated to the electrode terminal portion such as the bump electrode but is dispersed all over the surfaces of the semiconductor chips and, therefore, can be absorbed evenly by the surfaces, thus improving the reliability of the device.
Also, if the gap between the first and second semiconductor chips joined each other is filled with an insulating resin layer with a thermal shrinkage factor of 4% or less, even though both semiconductor chips are adhered to each other at a temperature of 300xc2x0 C. or so and then cooled down to the room temperature, they do not shrink than the electrode terminal connecting portion such as the bump electrode, so that the compressing force is not concentrated to the electrode terminal portion but is dispersed all over the surfaces of the semiconductor chips and, therefore, can be absorbed evenly by the surfaces, thus improving the reliability of the device.
Preferably, a circuit element is formed in the semiconductor layer at the joining portion of at least one of the first and second semiconductor chips, thus improving the integration density. That is, by the present invention, the chips can be joined to each other at a relatively low temperature with almost no pressure applied thereon, thus making it possible to form the circuit element even under the electrode pad or wiring joining portions.
A method for manufacturing a semiconductor device of the present invention, in which a first semiconductor chip or substrate and a second semiconductor chip are joined to each other with the surfaces thereof on which an electrode terminal or a wiring are formed respectively as facing each other via metals of the surface of the electrode terminal or the wiring, comprising the steps of; providing at least one of the metals with a low-melting point metal layer having a lower melting point than that of each of the metals, and melting the low-melting point metal layer or alloying the metals with the low-melting point metal layer to thereby join the first semiconductor chip or substrate and the second semiconductor chip to each other.
Another method for manufacturing a semiconductor device of the present invention, in which a first semiconductor chip or substrate and a second semiconductor chip are joined to each other with the surfaces thereof on which an electrode terminal or a wiring are formed respectively as facing each other via metals of the surface of the electrode terminal or the wiring, comprising the steps of; providing at least one of the metals with a low-melting point metal layer having a lower melting point than that of each of the metals, and liquefying the low-melting point metal layer to thereby diffuse the metals provided on the surface of the electrode terminal or the wiring into the liquefied low-melting point metal, by the liquid-phase diffusion method, thus joining the first semiconductor chip or substrate and the second semiconductor chip to each other.
Also, it is preferable that the metals are made of Au and the low-melting point metal layer is made of Sn or an Auxe2x80x94Sn alloy and also the first semiconductor chip or substrate and the second semiconductor chip are superposed one on the other with the electrode terminals or the wirings thereof as facing each other and heated to a temperature at which the Auxe2x80x94Sn alloy or Sn melts, to be self-aligned and joined with each other. That is, when these portion can be joined to each other by use of the Auxe2x80x94Sn alloy and heating them to a temperature of 280xc2x0 C. or so, they melt completely and, therefore, needs no pressure application nor complete alignment, to be brought up by surface tension to the joining position such as the bump in self-alignment.
Alternatively, further including the steps of; alloying the metals provided on the surface of the electrode terminal or the wiring of one of the first and second semiconductor chips with the low-melting point metal layer provided on the surface thereof, and joining to the other of the first and second semiconductor chips or substrate.